1. Field of the Invention
The present invention relates to a power supply apparatus.
2. Description of the Related Art
Recently, in response to a demand in decreasing power consumption of electric home appliances, business machines or the like, for example, power supply apparatuses are required to have high conversion efficiency. Among the power supply apparatuses, a switching power supply apparatus in which a power factor correction circuit (simply referred to as a “PFC” hereinafter) and an LLC resonant converter (simply referred to as an “LLC” hereinafter) are connected in series has been widely spread because of high conversion efficiency and low noise.
The PFC used for the switching power supply apparatus has a circuit structure including a boost switching converter, and is controlled to vary a switching frequency or ON/OFF duty ratio of a switching element in accordance with an Alternating Current (AC) input voltage waveform input from an AC power supply. For example, the PFC controls to have a waveform of a current flowing through the PFC become sinusoidal at a phase same as that of the sinusoidal AC input voltage waveform with respect to the AC input voltage waveform to improve power-factor (to reduce wattless power).
As an example of the PFC, an interleave PFC which is proposed for high conversion efficiency and low noise is known (see Japanese Laid-open Patent Publication No. 2007-195282, for example).
FIG. 1 is a view showing an example of such an interleave PFC 100. The interleave PFC 100 shown in FIG. 1 includes an AC power supply AC, a rectifier circuit RFY1, resistors R1 to R3, inductor elements L1 and L2, transistors Q1 and Q2 (switching elements), diodes D1 and D2, a capacitor element C1, an output voltage terminal Vout1, and a PFC controller 101.
In other words, the interleave PFC 100 shown in FIG. 1 includes two PFCs. The PFC controller 101 controls the transistors Q1 and Q2 of the two PFCs, respectively. The PFC controller 101 controls (multiphase-control) the transistors Q1 and Q2 of the PFCs by switching signals whose phases are different by 180°. With this structure, current continuously flows, noise can be reduced, and the maximum value of the flowing current is lowered so that low-loss (high conversion efficiency) can be actualized.
In the interleave PFC 100, the current flowing from the AC power supply (the voltage generated at the resistor R1 in FIG. 1) is detected and the current is controlled such that the integral value of the flowing current becomes sinusoidal in accordance with the sinusoidal AC input voltage waveform.
In the method disclosed in Japanese Laid-open Patent Publication No. 2007-195282, a composition of the outputs of the two PFCs is output from the output voltage terminal Vout1 of the interleave PFC 100. Thus, the two PFCs can have almost the same electric characteristics such that the output voltages of the two PFCs become substantially equal and only the phases are different. In such a case, there is no problem in having the ON/OFF duty ratio of the switching signals the same for the two PFCs. Thus, a control signal of one of the PFCs is used as a reference signal and the control signal is used for a control signal of the other PFCs by shifting the phase.
However, for a case when plural PFCs are used but output voltages of the plural PFCs are different, it is necessary to vary the ON/OFF duty ratio of the switching signals for the plural PFCs. In such a case, control is not performed similar to the way as described in Japanese Laid-open Patent Publication No. 2007-195282.